<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>封装ajax</title>
</head>
<body>
    <script>
         function ajax(url){
        return new Promise((resolve,reject)=>{
            let xhr =new XMLHttpRequest()
            xhr.open('get',url)  //链接
            xhr.send()  //发送资源
            xhr.onreadystatechange = function(){//进行动态监听
                console.log(xhr);
                if (xhr.readyState == 4){
                    if(xhr.status >= 200 && xhr.status < 300){
                        let res = JSON.parse(xhr.responseText) // 从后台获取json字符串，然后通过JSON.parse函数转换成json对象 JSON.strinfy()
                        resolve(res)
                    }else{
                        reject(xhr.responseText)
                    }
                }
            }
        })
    }

    ajax('./01.json').then((res)=>{
      console.log(res);
    })
    .catch((error)=>{
      console.log(error)
    })
    </script>
</body>
</html>